Printed circuit boards, or PCBs, are generally used to mechanically support and electrically connect electronic components using conductive pathways, or traces etched from copper sheets laminated onto a non-conductive substrate. A PCB populated with electronic components is referred to as a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). PCBs are generally rugged, inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Some PCBs have trace layers inside the PCB and are called multi-layer PCBs, and may be formed by bonding together separately etched thin boards. Some multi-layer PCBs may include several layers (e.g., 4 layers, 12 layers, 24 layers, or more).
Holes are typically drilled through a PCB with tiny drill bits (e.g., made of solid tungsten carbide) and/or LASERS in order to connect components to different layers of the PCB. The drilling may be performed by automated drilling machines, with the placement of the holes controlled by a drill tape or a computer generated drill file. The drill file describes the location and size of each hole to be drilled in the PCB. These holes are generally referred to as “vias.” These vias are often plated with conductive material (e.g., copper or aluminum) forming annular rings, which allow the electrical and thermal connection of conductors on opposite sides of a PCB.
It is also possible with controlled-depth drilling, laser drilling, or by pre-drilling the individual sheets of the PCB before lamination, to produce holes that connect only some of the copper layers, rather than passing through the entire board. These holes are called “blind vias” when they connect an internal copper layer to an outer layer, or “buried vias” when they connect two or more internal copper layers and no outer layers. The walls of the vias, for boards with 2 or more layers, are generally plated with copper to form plated-through-holes (PTH) that electrically connect the conducting layers of the PCB.
After the printed circuit board (PCB) is completed, electronic components must be attached to the PCB to form a functional PCBA. In through-hole construction, electronic component leads are inserted in PTHs in the PCB. In surface-mount technology (SMT) construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the PCB with molten metal solder.
Through-hole technology, also spelled “thru-hole”, refers to the mounting scheme used for pin-through-hole electronic components that involves the use of pins on the components that are inserted into copper PTH drilled in printed circuit boards (PCB) and soldered to pads on the opposite side. From the second generation of computers in the 1950s until surface-mount technology became popular in the late 1980s, every component on a typical PCB was a through-hole component. While through-hole mounting provides strong electrical and mechanical bonds when compared to surface-mount technology techniques, the additional drilling required makes the boards more expensive to produce. They also limit the available routing area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all layers to the opposite side. To that end, through-hole mounting techniques are now usually reserved for bulkier components such as electrolytic capacitors, power supplies, or semiconductors in larger packages that require the additional mounting strength provided by PTH technology.
PTH electronic components may be attached to a PCB using a soldering technique referred to as wave soldering. Wave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB. The process uses a tank to hold a quantity of molten solder, and the components are inserted into or placed on the PCB and the loaded PCB is passed across a pumped wave or fountain of solder. The solder “wets” the exposed metallic areas of the board (e.g., those not protected with solder mask, a protective coating that prevents the solder from bridging between connections), creating a reliable mechanical and electrical connection. The process is much faster and can create a higher quality product than manual soldering of components. Wave soldering is used for both through-hole printed circuit assemblies and surface mount assemblies.
While there are many types of wave solder machines, the basic components and principles of these machines are generally the same. A standard wave solder machine includes three zones: the fluxing zone, the preheating zone, and the soldering zone. An additional fourth zone, a cleaning zone, may also be used depending on the type of flux applied.
When a PCB enters the fluxing zone, a fluxer applies flux to the underside of the board. Two types of fluxers are used: a spray fluxer and a foam fluxer. For either flux application method, precise control of flux quantities is required. Too little flux will cause poor joints, while too much flux may cause cosmetic or other problems. Also, as can be appreciated, the types of flux may affect the end result.
The PCB will then enter the preheating zone. The preheating zone consists of convection heaters, which blow hot air onto the PCB to increase its temperature. Generally, preheating is necessary to activate the flux, and to remove any flux carrier solvents. Preheating is also necessary to prevent thermal shock, which may occur when a PCB is suddenly exposed to the high temperature of the molten solder wave.
The tank of molten solder has a pattern of standing waves (or, in some cases, intermittent waves) on its surface. When the PCB is moved over this tank, the solder waves contact the bottom of the board, and stick to the solder pads and component leads by surface tension. For the pins of PTH components, molten solder fills the holes around the pins by capillary action. Precise control of wave height is required to ensure solder is applied to all areas but does not splash to the top of the board or other undesired areas. This process is sometimes performed in an inert gas nitrogen (N2) atmosphere to increase the quality of the joints.
As the thickness of a PCB increases (e.g., above 100 mils, 150 mils, 200 mils, or more) and the mass of copper sheets increases (e.g., above 0.5 oz, 1.0 oz, 1.5 oz, 2.0 oz, or more), it may become more difficult to successfully fill the pin holes during the soldering process. One cause of the increased difficulty is that the molten solder tends to cool (“freeze”) prematurely before it has traveled from the bottom of the PCB to the top. This problem can be further exaggerated in pin holes that are used for ground and power connections. The reason for this is that a multilayered PCB may include several ground or power layers (e.g., 4 layers, 8 layers, 12 layers, or more) that include large sheets of copper. The multiple layers of copper sheets may conduct heat away from the molten solder (i.e., act as heat sinks), causing the solder to freeze prematurely and causing the pin hole to be only partially filled with solder (e.g., 75% filled, 50% filled, or less). When the pin hole is only partially filled with solder, the mechanical and electrical integrity of the solder connection may be significantly reduced or may even be ineffective. In this regard, standards have been set to require a minimum amount of solder that fills a through hole for various components. For example, the Institute for Interconnecting and Packaging Electronic Circuits (IPC) requires solder to fill at least 75% of the through hole for a signal pin and at least 50% of the through hole for a ground or power pin.
FIGS. 1 and 2 illustrate top and cross-sectional views of a PCB 100 that includes PTH components. The PCB 100 is configured with a resistor 104 and an integrated circuit 106. The PCB 100 includes a plurality of plated through-holes (e.g., PTH 108 and 110) that may be used to couple electronic components (e.g., the resistor 104 and the integrated circuit 106) from the top layer 102 of the PCB 100 to a conductor (not shown in FIG. 1) on the bottom surface of the PCB 100. The PCB 100 may also include a plurality of metal traces (e.g., a copper trace 111) that are operative to couple different components of the PCB 100 together.
FIG. 2 illustrates a cross-sectional view of a portion of the PCB 100 shown in FIG. 1 cut at the line 2-2. As shown, the PCB 100 includes a plurality of dielectric layers 102, 116, 120, 122, 126, 128, and 132. The PCB 100 also includes a plurality of conductive layers 108, 114a-c, 118, 124, 130, and 134 disposed between (or outside of) the dielectric layers (e.g., the conductive and dielectric layers alternate). In the example shown, the resistor 104 is coupled to the conductive layers 108 and 134 (e.g., ground layers or power layers) at the bottom of the PCB 100 by soldering a pin 105 of the resistor 104 to the PCB 100 using a PTH 103 partially filled with solder. Further, the resistor 104 is coupled to other internal conductive layers 114a-114c, which are the same type of layer as the layers 108 and 134 (e.g., ground layers or power layers).
As shown, to mechanically and electrically couple the resistor 104 to the PCB 100, solder 107 is used to connect the pin 105 to the PTH 103. Thus, the pin 105 is coupled via the solder 107 and the PTH 103 to the conductive layers 108, 114a-c, and 134. It is noted that the conductive layers 118, 124, and 130 do not contact the PTH 103 and are therefore not connected to the pin 105. In this regard, the layers 118, 124, and 130 may include signal layers, ground layers, or power layers that are connected to other components.
As shown, the solder 107 only partially fills the opening of the PTH 103. This may be due to the heat sinking effects caused by the ground or power layers 114a-c and 134 that are coupled to the PTH 103. That is, during the soldering process, molten solder 107 fills the opening of the PTH 108 from the bottom to the top via capillary action, losing heat in the process. If the molten solder 107 cools too rapidly, it may freeze prematurely, causing the opening in the PTH 103 to be only partially filled as shown. Since the PTH 103 is coupled to potentially large sheets of copper (e.g., the ground or power layers 114a-c) which have a high heat transfer coefficient, the heat of the molten solder 107 is dissipated rapidly through these electrical and heat conducting layers.